You should have enough minimum gas overall to get both you & your buddy to the surface. Unless you're diving solo it's not just about you. Having three second stages, two on your backgas and one on your pony, isn't the least bit complicated if everything's routed properly.
Every additional piece of equipment that you hang on yourself complicates your rig. Any piece of equipment that is not needed is no more than a liability. There is no foreseable situation in which you would need to be breathing off of both second stage regulators from the same first stage if you are carrying a pony.
I would love it if you explained these "incidents", I would assume the problem is that they switch to a pony reg when the valve isn't opened on the tank. With a slung tank this isn't a problem at all as it takes only seconds to turn the tank on; it's a serious issue if they can't reach the tank valve. It's easy to avoid this problem, about as easy as avoiding tripping while stepping onto a curb.
You may think that you can rationally think through a set of options and pick the best action in an emergency but in a real situation that rarely happens. It is very important to simplify and limit the number of choices one has to optimize the probability that the correct one is chosen in a stressful situation.
I will relate a couple of incidents I was present for. I was not underwater with these divers but I debriefed them after the fact.
In one instance a diver was getting ready to make a 130 foot dive. He had a 95CF main tank and a 30CF pony mounted to the side of his main tank. He had a regulator with two second stages and a gauge console on each tank. I suggested to him that he might want to remove the extra second stages and the console on the pony before going in. He declined, saying that he knew where everything was and had a system to determine which regulator was which. He was a very experienced diver, with a couple of hundred dives in his log.
He jumped into the water and was back up five minutes later. When we pieced the story together this is what we determined had happened:
He went in the water breathing off of his pony bottle, thinking it was his primary. He ran out of gas shortly after reaching the bottom and switched to his pony bottle regulator. Of course, he had already depleted his pony bottle so when he tried to take a breath he got nothing. Now he was on the verge of panic. He grabbbed the regulator from his buddy's mouth. His buddy, an excellent diver, easily dealt with the problem and switched to his own back-up. The two divers then took a couple of minutes to calm down and then made an air-sharing ascent. Back on the boat, we took the valve off of the pony bottle and puured about a litre of water out of it.
Another time I witnessed a diver go on a 130 foot dive with a similar set-up except that he had only one second stage on his pony. This time the diver went into the water breathing off of the proper regulator but got into trouble when he had an equipment problem on the bottom. He developed a free-flow with his primary regulator. He breathed it down a bit and then switched to what he thought was his pony. Of course, he made the wrong choice and put his octo in his mouth by mistake. He was already stressed out and when, just a few seconds later, he ran out of gas he panicked. He attempted to get air from his buddy but the exchange went bad because of his agitated state. His buddy was unable to control the situation and the diver ended up making a rapid ascent. He was not injured but could very well have been.
Those are just two examples. I have seen others.
You can dive with only one second stage on each tank if you want, but arguing that leaving the octo on the main tank is a safety hazard is just absurd. With smartly routed hoses having the third second stage gives you more options in an air-sharing situation.
With only exception (a knife or cutting tool) I have never witnessed an accident or near accident caused by a diver not having enough gear. I have seen many caused by divers haivg too much gear or inappropriate gear.
